Important links between exposure to tobacco marketing and tobacco use in youth have previously been uncovered. However, previous longitudinal research on tobacco marketing effects has not focused on particularly vulnerable populations or on youth during transitions in later adolescence when tobacco use experimentation often escalates and consolidates into life-long habits. Previous longitudinal research also has not investigated likely explanatory processes (intervening cognitive processes/mediators) through which marketing exposure has effects on tobacco use trajectories. The present project focuses on a vulnerable, lower SES, multi-ethnic (primarily Hispanic and non-Hispanic White) population of youth with substantially higher rates of smoking than the national norm, during ages in which dramatic changes in tobacco habits occur (16 to 20). Something must explain the substantially higher rates of tobacco use escalation in this population, and tobacco marketing at retail outlets in lower SES neighborhoods is one strong possibility. This project also evaluates key credible explanatory processes, well grounded in basic research across several disciplines. Some of the most credible processes that can explain marketing effects, not previously studied in this domain, have received consistent support from multiple lines of independent evidence across neural, cognitive, and behavioral levels of analysis --not merely survey or psychometric evidence. The study of marketing effects in a vulnerable population over critical ages, coupled with research on some of the most credible explanatory processes, will provide clear regulatory guidance to the FDA about any needs for new regulations on point-of- sale tobacco marketing (POS). Regulatory changes would be strongly suggested by firm evidence that POS has especially strong, explainable effects in lower SES neighborhoods and vulnerable populations. To investigate these effects and credible processes this project implements a cohort-sequential design in 1000 vulnerable youth from a multi-ethnic population in southern California. Three design features in this proposal are especially important to provide strong inference for any link between POS tobacco advertising and tobacco use, which is needed to support FDA policy decisions. First, the design allows the most rigorous testing of the temporal relationship among the variables, to date, one of the key criteria for causality. Second, testing for the intervening variables will provide support for plausible causal mechanisms also buttressed by extensive basic research. Finally, potential confounders and covariates are comprehensively assessed and thoroughly evaluated. The project provides the most rigorous, potentially most informative study to date about tobacco marketing effects in older youth. Results are likely to reveal why some older adolescents have substantially higher smoking rates (more than double the national average), leading to critical health risks in the future. Thus, the project has a strong potential for major regulatory implications for protecting the public health in adolescents. 1